A few weeks ago a reader in Biddeford wrote in to ask how to determine a bald eagle’s age.

“My wife and I have a camp on Etna pond. We where watching three bald eagles last Friday as they fed on a road kill deer carcass, that someone had put out on the ice,” he said. “Two of the eagles were adults with the full adult plumage. The third one was a juvenile…I know that the juveniles don’t get their adult plumage until they are several years old, but I was wondering when they achieve their full adult size. Is there any easy way to tell how old a juvenile is?”

This was a great question. I know bald eagles don’t attain their adult plumage until they are 4 years old, but hadn’t a clue as to how to tell 1-year, 2-year, and 3-year-olds apart. This was a good question to research, and it came at a good time – for now the eagles are beginning to build nests in preparation for breeding and raising young.

Populations of bald eagles across North America have varied migration patterns. Generally, an eagle will migrate when food becomes unavailable or if the climate becomes too severe. Southern populations may not migrate at all, while northern birds may to some extent. Those in Maine may either migrate south along the coast or inland along the Appalachian Mountains, according to the “Birds of North America” species account. Now, however, breeding birds are coming back to summer territories, often returning to the same nest they’ve used in previous years.

Such old nests came reach incredible sizes. The largest nest on record was located in St. Petersburg, Fla., measuring almost 3 meters in diameter and more than 6 meters tall. A famous nest in Ohio, used for an incredible 34 years, had similar proportions and weighed in at an estimated 2 metric tons. Apparently, it was also shaped like a wineglass!

Bald eagles have spectacular courtship displays, one of which is known as the “cartwheel display.” A pair will fly to a great height, lock talons, and tumble earthward, only just breaking contact at the last moment to avoid smashing into the ground. Another is the “roller-coaster” flight, in which an individual eagle will again fly to a great height and dive to earth, swooping up at the last moment before reaching the ground.

Nest building (or renovating) may begin between 1-3 months before egg laying. Both parents participate in nest construction as well as incubation, which generally lasts for 35 days. The young may fledge anywhere from eight weeks to 14 weeks of age and continue to follow and be fed by adults for another six weeks.

Here is where we get to the question of plumage differences with age. “The Sibley Guide to Birds” illustrates this nicely: juvenile, or first-year, birds are dark overall, with a slight, pale wash to their backs and tail feathers. Second-year birds show extensive white mottling on their wings and belly, appearing as if someone has splashed them with white paint. Also, 2-year-olds will have a well-defined white triangular-shaped patch on their backs, just behind their heads. Their bills, which are completely dark at one year, are now starting to lighten. Three-year-olds also still show white mottling on their bodies, but now their white head feathers are coming in; this contrasts with a dark, osprey-like eye-stripe. The beak continues to turn more yellow, with a black tip. Four-year-olds have acquired the adult plumage of dark body feathers and white head and tail feathers, and a yellow bill.

For a wonderful opportunity to observe a pair of eagles at a nest, go to the Biodiversity Research Institute’s Web site at http://www.briloon.org/ed/eagle and view them through a live “eagle cam.” The BRI is a nonprofit ecological research group located in Gorham.

NEWS bird columnist Chris Corio can be reached at bdnsports@bangordailynews.net